Synthesis, Characterization and Surface Attachment of Square Mixed-Valence Complexes as Building Block for Molecular Quantum Cellular Automata

Quantum-dot cellular automata (QCA) were proposed as a new architecture for computation, encoding binary information in the charge configuration of a cell with quantum 'dots'. Molecular QCA cells could be mixed-valence compounds, in which the metal redox sites play the role of dots and the tunneling paths are provided by bridging ligands. A suitable building block for constructing QCA circuits is a square of four electronically coupled dots containing two mobile electrons. Based on the general ideas of building the square compounds, a series of square compounds are designed. Synthetic approaches to the square compounds using molybdenum propiolate as a core linker failed possibly due to radicals generated in the process of the reactions. Alternatively, a known square with four ferrocenyl groups, {CpFe(η5-C5H4)}4(η4-C)CoCp (18), was investigated as a possible QCA cell, and its structure and dynamic NMR were measured. In contrast to the original work, this square compound does display four reversible waves in cyclic voltammetry and square wave voltammetry corresponding to five oxidation states of the compounds. By chemical methods, compound 18 can be oxidized to the monocationic and dicationic mixed-valence compounds, each of which are fully characterized by X-ray diffraction, FT-IR, EPR, magnetic susceptibility measurement and Mšssbauer spectra. Both mixed-valence compounds display intervalence charge transfer (IVCT) bands in near-IR region. Based on the analysis of the IVCT bands, it is concluded that the monocationic mixed-valence compound belongs to Class-II and the dicationic mixed-valence compound is Class II-III. The electron hopping frequency of the dicationic compound in acetone is estimated to be 107.5 s-1, which is appropriately fast for QCA application. Although a strategy for the attachment of mixed-valence compounds via electrostatic interaction is proposed, the experimental results (XPS and CV) prove that only neutral compounds are bound on the surface via Van de Waals forces. Moreover, due to the existence of silver ion on the surface, reversible waves in CVs of films were not obtained. Conclusive answers about the properties of the compounds on a surface substrate could not be drawn.